1. Field of the Invention
The present invention relates to multi-axis positioning equipment and, more particularly, to a zero-backlash, elevation-over-azimuth, two-axis positioner that is capable of continuous 360.degree., rotation in azimuth and limited rotation (130.degree., for example) in elevation.
2. Prior Art
A pair of patents will be mentioned that are exemplary, but not exhaustive, of the prior art in the field with which the present invention is concerned. U.S. Pat. No. 4,580,461 to Sears et al. discloses a gimbal mounting arrangement which provides at least two rotational axes about which a load such as an antenna or optical transducer may be gimbaled. The gimballing arrangement permits both axes of rotation to intersect at a point within the load being gimbaled thereby minimizing the rotational inertia of the load and of the gimbal mechanism itself. Because the rotational inertia is minimized, relatively rapid scan rates can be achieved using only relatively low powered drive inputs. U.S. Pat. No. 5,875,685 to Storaasli discloses a positioner including an output platform and a two-axis bearing suspension which permits the output platform to rotate about two orthogonal axes. The output platform is positioned by a pair of actuators which are mounted, as is the bearing suspension, to a positioner base. The actuators include bevel gears which engage ratchets to couple the actuators and the platform.
Most existing elevation-over-azimuth, two-axis positioners have two separate drives for the azimuth and elevation axes. The elevation axis drive is usually located on the far side of the azimuth axis from the stationary base, requiring an electrical slipring to allow for continuous azimuth rotation. The slipring is typically a very expensive component. Many known constructions of our elevation-over-azimuth, two-axis positioners use small, high-speed electric motors with geartrains to drive the low-speed output axes. Geartrains have one major disadvantage--backlash. Excessive backlash can cause problems with stability in a closed-loop control system, and backlash adds to position error.
Existing elevation-over-azimuth, two-axis positioners in which the position of the elevation axis is determined by the position of the elevation drive in the base relative to the position of the azimuth drive in the base have a major disadvantage. This is that any backlash in the azimuth drive would add to the backlash in the elevation drive, increasing the total backlash at the elevation output axis.
Direct drive electric motors have been used to eliminate backlash. They generally require more size, mass, and input power than a small, high-speed motor with a geartrain for a given output power with a slow-moving output axis.
Many methods have been used for control of backlash in geartrains. George W. Michalec's book Precision Gearing: Theory and Practice, published in 1966, has a good description of many different methods. Most have disadvantages such as increased size, weight, and cost. Examples are split, spring-loaded scissor gears and the use of auxiliary geartrains.
It was with knowledge of the foregoing that the present invention has been conceived and is now reduced to practice.